This invention relates generally to the handling of materials used in the electronics field. More particularly, this invention relates to a technique of forming a low temperature metal bond to facilitate the transfer of bulk and thin film materials used in the electronics field.
The intimate integration of thin film materials with disparate properties is required to enhance the functionality of integrated Microsystems. For example, combining laser diodes with low cost electronics necessitates the integration of III-V semiconductors with silicon. The materials integration can be done simply by direct deposition of the thin film onto the final substrate. In many cases, however, direct deposition involves substantial sacrifices in the micro-structural quality, properties and performance of the thin film. In some cases, such as integration of piezoelectric electro-ceramic thin films with polymer substrates, the processing conditions (e.g., temperature and ambient) preclude direct deposition. In these instances, a bonding and lift-off approach may be required.
A bonding process involves adhering a heterostructure (e.g., a thin film) still on its growth substrate to a final micro-machined, patterned, and metallized substrate. To allow maximum flexibility for a wide range of material combinations it would be highly desirable to adhere to a number of constraints. For example, the bonding layer should have low resistance to shear stress at temperatures below 200xc2x0 C. such that submicron surface asperities and particulates do not prevent full surface contact. It would also be desirable to have a bonding layer that is both adherent to the heterostructure and the final acceptor substrate. In addition, the bonding process should be performed in such a manner that it does not leave behind a low melting point phase or residue. Ideally, the resulting bond would have low electrical resistance and low thermal resistance. Finally, the bonding layer should be thinner than the thin film to be transferred, such that its properties do not dominate those of the transferred thin film.
If the foregoing constraints could be satisfied, the opportunities for integration of thin film materials with other systems would increase. More particularly, opportunities for the direct integration of thin films with novel substrate materials for improved device performance would increase.
A method of forming a low temperature metal bond includes the step of providing a donor substrate, such as a crystallographically oriented donor substrate, including a sapphire donor substrate or a MgO donor substrate. The donor substrate may also be quartz or fused silica. A thin film is grown on a surface of the donor substrate. The thin film may be an oxide, nitride or Perovskite. The invention may be implemented using nitride thin films, including AIN, GaN, InN, and all of their solid solutions, alloys, and multi-layers. An acceptor substrate is then produced. The acceptor substrate may be Si, GaAs, polymers, such as polyimide, or stainless steel for use in microrobotics. A multi-layer metal bond interface for positioning between the thin film and the acceptor substrate is then selected. The multi-layer metal bond interface must satisfy a set of criteria, such as low temperature bonding, low resistance to shear stress, of durability to adhere to the donor and acceptor substrates, and the ability to form a thin new bonded layer. A bonded layer is then formed, at a temperature below approximately 200xc2x0 C., between the thin film and the acceptor substrate using the multi-layer metal bond interface. The donor substrate is then severed from the thin film to isolate the thin film for subsequent processing.
An embodiment of the invention has utilized low temperature Pdxe2x80x94In metal bonding to bond GaN thin films onto Si, GaAs, and polyimide substrates. The Pdxe2x80x94In bonding layers form a PdIn3 compound after pressure bonding at a temperature of approximately 200xc2x0 C. Separation of the sapphire substrate from the GaN thin film can be achieved using a pulsed ultra-violet laser lift-off process. Thin film characterization by x-ray diffraction, scanning electron microscopy, and atomic force microscopy verify that the GaN retains its crystal quality before and after thin film separation and transfer. The technique of the invention creates opportunities for integration of GaN-based devices with other material systems. The xe2x80x9ccut and pastexe2x80x9d methodology of the invention may be used to combine GaN with other material systems that otherwise cannot be used in conventional growth processes. The technique of the invention allows direct integration of GaN with novel substrate materials for improved device performance.